This invention relates to a method and an apparatus for curing (hardening) lacquer layers applied to an article surface (substrate) of complex configuration, by means of highly accelerated, high-energy electrons which, by means of a scanner, are spread out in a fan-like beam and which are directed onto the article through an electron outlet window.
It is known to irradiate lacquer coatings by highly accelerated, high-energy electrons for the purpose of curing the lacquer. The electrons penetrate through opaque materials and therefore are capable of curing even pigmented lacquers. The ionizing radiation effects an interaction between the electrons on the one hand and the atoms of the irradiated layer, on the other hand, whereby the highly accelerated electrons transfer energy to the lacquer layer, especially to the double bonds of the lacquer molecules.
Curing of lacquers by means of electron irradiation has, among others, the advantage that the pigmented and transparent lacquer systems are cured uniformly and further, the curing periods are very short and thus, a high production output is achieved. Further, high-quality surfaces are obtained which, even in case of a thin layer, have superior filling properties.
The electrons are generated by an electron accelerator in which a cathode of an accelerating tube emits electrons under high potential difference and then the electrons are accelerated in a high-voltage electric field. In a downstream-arranged scanner the electrons are, by means of alternating fields, deflected back and forth and thus are spread into a fan-like beam bundle. Such an electron beam leaves the high vacuum through an at least substantially beam-transparent outlet window made, for example, of a particular titanium alloy, and then is deflected onto the article to be irradiated. During passage through the electron outlet window (which seals the electron accelerator and the scanner from the external environment and atmosphere pressure), one part of the radiation output of the electron beam is absorbed so that a minimum electron energy has to be maintained.
Conventionally, the electron beam curing is utilized in case of articles which have a flat, planar surface such as plates or foils, as described, for example, in an article by B. P. Offermann: "Lackhartung mit energiereichen Elektronen" (Curing of Lacquers with High-Energy Electrons), ETZ-B, Volume 23 (1971), Issue 25, or mentioned in an article in VDI-Nachrichten of May 11th, 1979 (page 1, last paragraph). Concerning the treatment of a polyethylene insulation of an electric cable (which thus is not a planar article), it is further known, as described, for example, in German Auslegeschrift (Published Accepted Patent Application) No. 1,046,789, to bring one or more particular metallic surfaces into the path of the electron beam in order to obtain a uniform irradiation of the insulation. The high-energy electrons are scattered on the metal surfaces and deflected in such a manner that they penetrate the cable insulation. For improving the uniformity of the irradiation of the cable, either an additional shield is provided which acts as a screen against the direct radiation of the high-energy electrons or the cable is rotated.